Achieving high p-type conductivity in many wide-bandgap semiconductors is difficult due to the large acceptor binding energies. In the case of III-V nitrides, the acceptor effective Rydberg energies are 200-400 meV for commonly used acceptors such as Mg and Zn. The free carrier concentration in the freeze-out regime (E.sub.a &gt;&gt;kT) in a semiconductor with an acceptor concentration N.sub.A and an acceptor binding energy of E.sub.a, is given by ##EQU1## where g is the acceptor degeneracy, N.sub.v, is the effective density of states at the valence band edge, and kT is the thermal energy. For activation energies of 200 meV, the electrical activation calculated from Eq. (1) is 6%.
A possible solution to this problem is to heat the semiconductor to higher temperatures thereby increasing the thermal energy, kT. As a result, more acceptors would be ionized. However, this solution is impractical since it is desirable to operate semiconductor devices at room temperature.
Another possible solution is to dope a superlattice structure with acceptors. Acceptor doping in a superlattice structure in wide-bandgap II-VI materials, such as ZnSe, is known in the art For instance, doping of acceptors in a superlattice structure to improve the activation ratio was proposed in an article by Suemune. Suemune, "Doping in a superlattice structure: Improved hole activation in wide-gap II-VI materials," J. Appl. Phys., Vol. 67, No. 5, Mar. 1, 1990. However, while the structures of Suemune exhibit high conductivity parallel to the superlattice planes, their conductivity is poor along a direction normal to the superlattice plains.